JP3607836B2 - Optical pickup device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical pickup device, and more particularly to an optical pickup device used in a magneto-optical disk reproducing device.
[0002]
[Prior art]
In recent years, a magneto-optical disk reproducing apparatus capable of repeatedly recording and reproducing data such as sound, images and documents has been developed. The optical pickup apparatus is a small component as a basic component of the magneto-optical disk reproducing apparatus. The emphasis is on making it easier.
[0003]
A magneto-optical pickup device disclosed in Japanese Patent Application Laid-Open No. 8-329544 is known as a magneto-optical pickup device that is miniaturized. With reference to FIG. 11 and FIG. 12, the first conventional magneto-optical pickup device is an optical module 35, a transparent substrate 41 placed on the optical module 35, and placed on the transparent substrate 41. It includes a polarizing prism 43 having a trapezoidal cross section obtained by joining a prism having a triangular cross section and a prism having a parallelogram cross section, and an objective lens 45 as a condensing element provided above the polarizing prism 43. A substrate 36 is provided inside the optical module 35, and a laser diode 37 and photodiodes 38 to 40 are formed on the substrate 36. As shown in FIG. 12, each of the photodiodes 38 and 39 is divided into six photodiodes 38a to 38f and 39a to 39f. The photodiode 40 is divided into two photodiodes 40a to 40b. A hologram diffraction element 42 is formed on the surface of the transparent substrate 41 facing the laser diode 37.
[0004]
The joint surface 43a between the triangular prism of the polarizing prism 43 and the parallelogram prism is set to have a transmittance and a reflectance of 70% and 30%, respectively, with respect to light (P-polarized light) emitted from the laser diode 37, and This is a polarization separation surface in which the transmittance of S-polarized light is set to 100%.
[0005]
A prism type analyzer 44 having a trapezoidal cross section in which a prism having a triangular cross section and a prism having a parallelogram cross section are joined is provided above the photodiode 40. The joint surface 44a of the two prisms is a polarization separation surface in which the transmittance of P-polarized light is set to 100% and the reflectance of S-polarized light is set to 100%. The joint surface 44a of the prism type analyzer 44 is provided above the photodiode 40a, and the reflection surface 44b of the prism type analyzer 44 is provided above the photodiode 40b.
[0006]
The basic operation of the above magneto-optical pickup device will be described. The P-polarized light emitted from the laser diode 37 passes through the transparent substrate 41 on which the hologram diffraction element 42 is formed, and enters the polarization separation surface 43 a of the polarizing prism 43. 70% of the incident P-polarized light passes through the polarization separation surface 43 a and is condensed on the magneto-optical recording medium 12 by the objective lens 45. The condensed light is reflected on the magneto-optical recording medium 12 by rotating the polarization plane of the light by 0.5 ° by the recorded magnetic signal and obtaining an S-polarized component which is a magneto-optical signal component. Then, the light passes through the objective lens 45 and returns to the polarization separation surface 43a. On the polarization separation surface 43a, 70% of the P-polarized component passes, and 30% of the P-polarized component and 100% of the S-polarized component are reflected. The reflected light is further reflected by the reflecting surface 43 b, passes through the transparent substrate 41, enters the optical module 35, and enters the polarization separation surface 44 a of the prism type analyzer 44. Of the light incident on the polarization splitting surface 44a, the P-polarized component passes through and is incident on the photodiode 40a, the S-polarized component is reflected, further reflected on the reflecting surface 44b, and then incident on the photodiode 40b.
[0007]
At this time, when the signals obtained by the photodiodes 40a and 40b are indicated using the same reference numerals, the magneto-optical signal RF is obtained by the following equation (1).
[0008]
RF = 40a-40b (1)
On the other hand, the light that has passed through the polarization splitting surface 43a is incident on the hologram diffraction element 42 and is diffracted at a diffraction angle of 5 ° to 20 °. Each incident. The hologram diffractive element 42 has a lens effect, and the + 1st order diffracted light is focused closer to the hologram diffractive element 42 than the photodiode 38, and the −1st order diffracted light is focused farther than the photodiode 39. When the magneto-optical pickup device and the magneto-optical recording medium 12 are in focus, the diameters of the light spots on the photodiode 38 and the photodiode 39 are the same, so the focus error signal FE is expressed by the following equation (2 ).
[0009]
FE = {(38a + 38c + 38d + 38f) + (39b + 39e)}-{(38b + 38e) + (39a + 39c + 39d + 39f)} (2)
The dividing lines for dividing the photodiodes 38a to 38c and the photodiodes 38d to 38f, the dividing lines for dividing the photodiodes 39a to 39c and the photodiodes 39d to 39f, and the direction of the information track of the magneto-optical recording medium 12 are as follows. If the magneto-optical pickup devices are arranged so as to be parallel, the tracking error signal TE can be expressed by the following equation (3).
[0010]
TE = {(38a + 38b + 38c) + (39a + 39b + 39c)}-{(38d + 38e + 38f) + (39d + 39e + 39f)} (3)
Thus, by providing the polarizing prism 43 having different reflectance and transmittance for the P-polarized light and S-polarized light integrally with the optical module 35, the magneto-optical pickup device can be miniaturized and the light utilization efficiency is increased. A sufficient S / N (Signal / Noise) ratio of the magnetic signal can be ensured.
[0011]
JP-A-8-329544 discloses a magneto-optical pickup device as shown below.
[0012]
13 and 14, the conventional second magneto-optical pickup device is different from the first magneto-optical pickup device in that the polarizing prism 43 is polarized between three triangular prisms having a triangular cross section. The configuration in which the hologram 54 is sandwiched and the photodiodes 38 and 39 disposed on both sides of the hologram diffraction element 42 and the laser diode 37 are eliminated, and the 0th order light, the first order light, This means that the photodiodes 55, 56 and 57 are arranged at the light receiving position of the −1st order light, respectively.
[0013]
The polarization hologram 54 is made of lithium niobate, and separates incident light into two linearly polarized light components orthogonal to each other, with one linearly polarized light component being zero-order light and the other linearly polarized light component being ± primary light. There is an action to inject.
[0014]
The polarization hologram 54 has a lens effect such that the focal positions of ± first-order diffracted light are different. The + 1st-order light is focused closer to the polarization hologram 54 than the photodiode 56, and the −1st-order light is a photo It is configured to focus at a position farther from the diode 57, and is configured so that the 0th-order light does not focus on the photodiode 55. For this reason, since the diameters of the light spots on the photodiodes 56 and 57 of ± primary light are different, the sizes of the photodiodes 56 and 57 are changed so that a focus error signal can be obtained correctly. The photodiodes 56 and 57 are each composed of three photodiodes 56a to 56c and 57a to 57c divided in a direction perpendicular to the arrangement direction. The photodiode 55 includes photodiodes 55a and 55b divided in the same direction as the arrangement direction of the photodiodes 56 and 57.
[0015]
Since the basic operation of the second magneto-optical pickup apparatus is the same as that of the first magneto-optical pickup apparatus, description thereof will not be repeated.
[0016]
In the above configuration, the magneto-optical signal RF is represented by the following equation (4).
RF = (55a + 55b) − {(56a + 56b + 56c) + (57a + 57b + 57c)} (4)
Further, the focus error signal FE and the tracking error signal TE are expressed by Expression (5) and Expression (6), respectively.
[0017]
FE = (56a + 56c + 57b) − (56b + 57a + 57c) (5)
TE = 55a-55b (6)
Therefore, according to the second magneto-optical pickup device, as in the first magneto-optical pickup device, a sufficient S / N ratio of the magneto-optical signal can be obtained, and the small size and low cost with high reliability and durability can be obtained. A magneto-optical pickup device is obtained. Further, in the optical path from the laser diode 37 to the magneto-optical recording medium 12, there is no extra optical branching element such as a diffraction grating other than the polarizing prism 43. For this reason, the light utilization efficiency is further improved. At the same time, the area of the photodiode on the substrate 36 can be reduced by detecting the magneto-optical signal RF, the focus error signal FE, and the tracking error signal TE with the common photodiodes 55 to 57. For this reason, further miniaturization and cost reduction of the magneto-optical pickup device can be realized.
[0018]
[Problems to be solved by the invention]
However, in the first magneto-optical disk pickup apparatus, the hologram diffraction element 42 is disposed between the laser diode 37 and the magneto-optical recording medium 12. For this reason, the optical power reaching the magneto-optical recording medium 12 decreases due to the light diffraction phenomenon. Therefore, the output of the laser diode 37 must be increased.
[0019]
Further, when light emitted from the laser diode 37 and further diffracted by the hologram diffraction element 42 enters the objective lens 45, the incident light is reflected by the magneto-optical recording medium 12 and projected onto the photodiodes 38 and 39. . For this reason, false signals are generated in the focus error signal and the tracking error signal. In order to avoid this problem, the formation region of the hologram diffraction element 42 may be limited as in the optical pickup device disclosed in JP-A-2-273336 (JP-B-7-3703). The wavelength of the hologram diffractive element 42 also decreases in proportion to the shorter wavelength. For this reason, it becomes difficult to manufacture a magneto-optical disk pickup device at low cost.
[0020]
Further, the photodiodes 38 and 39 are disposed in the vicinity of the laser diode 37. For this reason, the radiated light from the laser diode 37 reflected by the transparent substrate 41 enters the photodiodes 38 and 39, and a false servo signal is likely to be generated.
[0021]
Furthermore, the width of the polarizing prism 43 is 2 to 5 mm for reasons of arrangement and manufacture. For this reason, the distance from the photodiode 38 to the photodiode 40 is also 2 to 5 mm, and the substrate 36 having a size 2 to 5 times the normal size is required to form the photodiodes 38 to 40 on the substrate. This increases the manufacturing cost of the magneto-optical disk pickup device.
[0022]
On the other hand, in the second magneto-optical disk pickup apparatus, the hologram diffraction element 42 as shown in FIG. 11 does not exist on the optical path from the laser diode 37 to the magneto-optical recording medium 12. Therefore, unlike the first magneto-optical disk pickup device, the optical power reaching the magneto-optical recording medium 12 is not reduced by the light diffraction phenomenon. Further, a false signal does not occur in the focus error signal and the tracking error signal.
[0023]
Further, in the second magneto-optical disk pickup device, the photodiodes 55 to 57 are arranged at positions away from the laser diode 37, so that a false servo signal is generated as in the first magneto-optical pickup device. There is no need to do.
[0024]
However, also in the second magneto-optical disk pickup device, photodiodes 55 to 57 are formed on the substrate 36. For this reason, the substrate 36 having a size 2 to 5 times the normal size is required, and there is a common problem with the first magneto-optical disk pickup device that the manufacturing cost of the magneto-optical disk pickup device increases.
[0025]
Further, in the second magneto-optical disk pickup device, the polarizing prism 43 is composed of a number of members. For this reason, the positional deviation of the light beam incident on the photodiodes 55 to 57 becomes large, making assembly difficult. Further, there is a problem that the yield of the polarizing prism 43 is deteriorated.
[0026]
Further, the polarization separation surface 43a and the reflection surface 43b of the polarization prism 43 are formed of different members. For this reason, it is difficult to ensure the parallelism with high accuracy, and there is a problem in terms of mass productivity.
[0027]
Furthermore, the polarization hologram 54 made of lithium niobate has a diffraction efficiency of ± 1st order diffracted light at most 40%, so that it is emitted as linearly polarized light emitted as 0th order light and ± 1st order diffracted light. The difference in intensity from linearly polarized light is 1.2 times. For this reason, it is difficult to directly reproduce the magneto-optical signal by calculating the difference signal, and a part of polarized light to be emitted as the 0th-order light is diffracted or ± 1st-order diffracted light due to a hologram manufacturing error. In other words, the light to be emitted is emitted as zero-order light without being diffracted, so that a sufficient extinction ratio cannot be obtained, and the signal quality is deteriorated.
[0028]
The present invention has been made to solve the above-described problems, and an object thereof is to provide a small-sized optical pickup device.
[0029]
Another object of the present invention is to provide an optical pickup device with high light utilization efficiency.
[0030]
Still another object of the present invention is to provide an optical pickup device capable of improving the degree of design freedom.
[0031]
Still another object of the present invention is to provide an optical pickup device excellent in mass productivity and processing accuracy.
[0032]
[Means for Solving the Problems]
An optical pickup device according to an aspect of the present invention includes a light source, a lens disposed on an optical path from the light source to the magneto-optical recording medium, an optical path from the light source to the lens, and reflection from the magneto-optical recording medium. A beam splitter for separating a part of the light, and a photodetector for detecting the reflected light separated by the beam splitter;A first diffractive element disposed on the optical path from the beam splitter to the photodetector;The beam splitter is made of isotropic optical material and reflects light from the light sourceMagneto-optical recording mediumA first member for allowing reflected light from the magneto-optical recording medium to pass, and a magneto-optical recording medium which is adjacent to the first member and is made of an anisotropic optical material and has passed through the first member A second member for further passing reflected light fromThe first member has a parallel four-sided cross section having a first parallel surface facing each other and a second parallel surface facing each other and intersecting each first parallel surface at a predetermined angle. The first parallel surface is in contact with the second member, and the other of the second parallel surfaces faces the lens so that one of the second parallel surfaces faces the light source. As described above, each of the predetermined angles is emitted from the light source, and light incident on one of the second parallel surfaces at a predetermined incident angle is reflected on the other of the first parallel surfaces and the first parallel surface. And is selected to exit from the other of the second parallel surfaces, and the photodetector is parallel to a plane orthogonal to the first and second parallel surfaces of the beam splitter. The first diffractive element includes a set of two-divided photodetectors that are divided into two at the boundary line. Reflection from a magneto-optical recording medium having first and second regions divided into two by a dividing line parallel to a plane orthogonal to the first and second parallel surfaces of the liter, and diffracted in the first region The light is guided on the boundary line of the two-part photodetection unit.
[0033]
Branching of reflected light and polarization separation can be realized by a beam splitter composed of only two members (a first member and a second member). For this reason, an apparatus can be reduced in size. In addition, since the number of components of the beam splitter is small, the manufacturing error is small, and the positional deviation of the light beam incident on the photodetector is small. For this reason, the assembly of an apparatus is easy. Further, the birefringence of the crystal is used for polarization separation. For this reason, there is no difference in intensity between the two separated linearly polarized lights. Furthermore, since the extinction ratio is determined by crystallinity, an extinction ratio of about 1: 100 can be easily obtained by using a good crystal. For this reason, light utilization efficiency can be improved. In addition, since the LD mounting portion is unnecessary for the substrate of the photodetector, it can be made small, and the manufacturing cost can be suppressed.
The cross section of the first member is a parallelogram. For this reason, each parallelism and space | interval of the light which injects into a 1st member, and the light radiate | emitted from a 1st member can be managed with high precision. In addition, since the first member is a parallelogram, a manufacturing method in which a large substrate is overlaid and then cut can be adopted as a method of manufacturing the beam splitter. For this reason, the mass productivity of the optical pickup device can be improved.
The first diffractive element can diffract part of the reflected light from the magneto-optical recording medium and generate a control signal. The first diffraction element does not exist on the optical path from the light source to the magneto-optical recording medium. For this reason, since the light emitted from the semiconductor laser is efficiently transmitted to the magneto-optical recording medium, a semiconductor laser having a lower output can be used. Thereby, the downsizing of the apparatus and the degree of design freedom can be improved, and the mass productivity can be improved. Moreover, it is not necessary to consider that the diffracted light from the first diffraction element does not enter the lens. For this reason, the freedom degree of design of the 1st diffraction element improves. Furthermore, since the diffraction angle of the first diffraction element may be small, the grating pitch of the hologram can be kept large even if the wavelength of the incident light is shortened. Therefore, for example, the first diffraction element can be manufactured by a method such as a contact exposure method, mass productivity and processing accuracy can be improved, and an optical pickup device can be manufactured at low cost. Further, there is no problem that the light diffracted by the first diffractive element is reflected by the magneto-optical recording medium and incident on the photodetector to generate false signals in the focus error signal and the tracking error signal. .
[0034]
Preferably, the first member has a refractive index substantially the same as the extraordinary light refractive index of the second member.
[0035]
By selecting the first and second members so that the first member has a refractive index substantially the same as the extraordinary light refractive index of the second member made of an anisotropic optical material, The polarization separation angle at the surface in contact with the second member can be increased. Therefore, the height of the first and second members can be reduced. Further, since the influence of the walk-off can be suppressed, the optical design can be easily performed.
[0036]
More preferably, in the first member, the difference between the refractive index of the first member and the extraordinary light refractive index of the second member is 1 of the difference between the ordinary light refractive index and the extraordinary light refractive index of the second member. The refractive index is within / 2.
[0037]
By selecting the first and second members so as to have such a refractive index, the polarization separation angle at the surface where the first member and the second member are in contact can be increased. Therefore, the height of the first and second members can be reduced. In addition, the optical design can be relatively easily performed although the influence of the walk-off slightly occurs.
[0040]
More preferably, the crystal axis of the second member is orthogonal to the light emitted from the other of the second parallel surfaces, and the direction vector of the direction of the light emitted from the other of the second parallel surfaces and The plane is selected so as to form approximately 45 ° with respect to a plane including the one normal vector of the first parallel plane.
[0041]
The crystal axis of the second member, which is an anisotropic optical material, is orthogonal to the light emitted from the other of the second parallel surfaces and the light emitted from the other of the second parallel surfaces. It is selected to form approximately 45 ° with respect to a plane including the direction vector of the direction and the one normal vector of the first parallel plane. For this reason, the magneto-optical signal contained in the light condensed on the magneto-optical recording medium by the collimating lens and the objective lens can be stably separated.
[0042]
More preferably, a light-transmitting substrate provided between the light source and the photodetector and the beam splitterAnd the first diffraction element is provided on the light transmissive substrate..
[0044]
More preferably, it further includes a second diffractive element that is provided at a position in the light transmissive substrate that receives light from the light source and divides the light from the light source into three or more light beams.
[0045]
A second diffractive element is also formed on the light-transmitting substrate on which the first diffractive element is formed. For this reason, a tracking signal can be obtained by a stable three-beam method without increasing the number of parts. As a result, the apparatus can be miniaturized and the light utilization efficiency can be increased.
[0046]
More preferably, the first and second diffraction elements are juxtaposed on the same plane.
Since the second diffractive element is formed on the same plane as the first diffractive element, the second diffractive element and the first diffractive element can be manufactured simultaneously. For this reason, the number of manufacturing steps does not increase, and the mass productivity of the apparatus can be improved.
[0047]
More preferably, it further includes a half-wave plate provided between the light source and the beam splitter.
[0048]
A half-wave plate is disposed between the light source and photodetector and the beam splitter. Therefore, a pickup can be configured regardless of the far field pattern of the light source and the polarization direction, and a polarizing film formed on the boundary surface between the first and second members of the beam splitter can be easily produced.
[0049]
More preferably, the second member has a refractive index of 1.4 to 2.0.
A material having a refractive index close to that of glass is used as the second member. For this reason, the refraction angle of the chief ray at the boundary between the first member and the second member becomes small, and the generated astigmatism and coma aberration can be suppressed.
[0050]
More preferably, the second member is made of lithium tetraborate.
When lithium tetraborate oxide having a large birefringence is used as the second member, the spatial distance of the polarized light can be increased.
More preferably, the output of the two-divided detection unit is compared to obtain a focus error signal.
[0051]
DETAILED DESCRIPTION OF THE INVENTION
[Embodiment 1]
Referring to FIG. 1, an optical pickup device according to an embodiment of the present invention includes a stem 8, a semiconductor laser 1 that is a light source provided on the stem 8, a cap 9 that covers the stem 8, and a cap 9. A light-transmitting substrate 4 attached to the light-transmitting substrate 4, a half-wave plate 3 attached to the light-transmitting substrate 4, a beam splitter 2 attached to the half-wave plate 3, and the semiconductor laser 1 The collimating lens 10 and the objective lens 11 that collect the light emitted from the magneto-optical recording medium 12 and the stem 8 and the reflected light from the magneto-optical recording medium 12 branched by the beam splitter 2 are detected. And a photodetector 7 for
[0052]
The beam splitter 2 is disposed on the optical path from the semiconductor laser 1 to the collimating lens 10 and separates part of the reflected light from the magneto-optical recording medium 12 as described above. The beam splitter 2 is composed of a first member 15 made of an isotropic optical material and a second member 14 made of an anisotropic optical material, and the light from the semiconductor laser 1 is the first member 15. It passes through only the inside and reaches the collimating lens 10, and the reflected light from the magneto-optical recording medium 12 passes through the first member 15 and the second member 14 and reaches the photodetector 7. . The first member 15 constituting the beam splitter 2 is mainly a prism having a parallelogram in cross section, a first surface 18 in contact with the second member 14, and a second surface 16 adjacent to the first surface 18. And a third surface 17 facing the first surface 18 and a fourth surface 19 facing the surface of the second surface 16.
[0053]
The light transmissive substrate 4 is disposed on an optical path from the beam splitter 2 to the photodetector 7 and diffracts part of the reflected light from the magneto-optical recording medium 12 to generate a control signal. 6 is formed. Further, on the surface where the first diffractive element 6 is formed, the light emitted from the semiconductor laser 1 passes through the surface where the light from the semiconductor laser 1 passes through two tracking beams and one information reproducing beam. Are formed into a total of three beams.
[0054]
A window glass 21 is attached to the light passage region of the cap 9, and the inside is hermetically sealed. By sealing the inside of the package 13 composed of the stem 8 and the cap 9, the relative position between the semiconductor laser 1 and the photodetector 7 can be kept stable.
[0055]
The P-polarized light emitted from the semiconductor laser 1 is divided by the second diffraction element 5 into a total of three beams, two tracking beams and one information reproducing beam. The three beams are converted to S-polarized light by rotating the 90 ° polarization plane by the half-wave plate 3. The three beams converted to S-polarized light enter the first member 15 of the beam splitter 2 from the second surface 16. The three beams incident on the first member 15 are reflected by the third surface 17 and the first surface 18 and then emitted from the fourth surface 19, and are magneto-optical recording medium by the collimating lens 10 and the objective lens 11. 12 is condensed. As described above, the second diffractive element 5 is a linear grating formed on the same plane as the first diffractive element 6 of the light-transmitting substrate 4 and having a constant interval as shown in FIG. The polarization characteristics of the first surface 18 include, for example, a reflectance of S-polarized light of 70% (transmittance of S-polarized light is 30%), and a reflectance of P-polarized light is 0% (transmittance of P-polarized light is 100%). The magneto-optical recording medium 12 is irradiated with 70% of the light emitted from the semiconductor laser 1.
At this time, the second diffraction element 5 has a diffraction grating parallel to the X axis. For this reason, the two tracking beams and one information reproducing beam described above are arranged in parallel to the Z axis.
[0056]
The plane of polarization of the light reflected by the magneto-optical recording medium 12 rotates according to the direction of magnetization recorded on the magneto-optical recording medium 12. The light whose polarization plane has rotated passes through the objective lens 11 and the collimating lens 10 and enters the fourth surface 19 of the first member 15 of the beam splitter 2. The light incident on the fourth surface 19 passes through the first surface 18 and enters the second member 14. As described above, the polarization characteristics of the first surface 18 are, for example, that the reflectance of S-polarized light is 70% (the transmittance of S-polarized light is 30%), and the reflectance of P-polarized light is 0% (the transmittance of P-polarized light is 100). %). For this reason, when the light reflected by the magneto-optical recording medium 12 passes through the first surface 18, the amount of rotation of the polarization plane apparently increases. That is, as shown in FIG. 3, when the rotation amount of the polarization plane of the reflected light before passing through the first surface 18 is θ, the rotation amount of the polarization plane of the reflected light after passing through the first surface 18 is θ ′. (Θ ′> θ).
[0057]
Since the second member 14 has optical anisotropy, the reflected light from the magneto-optical recording medium 12 in the second member 14 is separated into two orthogonal polarization components and travels in different directions. The reflected light separated by polarization passes through the half-wave plate 3 and enters the first diffraction element 6, and a part of the reflected light is diffracted. Since three light beams are reflected from the magneto-optical recording medium 12, a total of six light beams are incident on the first diffraction element 6 by polarization separation.
[0058]
With reference to FIG. 4, the shape of the first diffraction element 6 and the six light beams incident thereon will be described. Solid and dashed circles indicate light separated into two orthogonal polarization components. The first diffraction element 6 isThe boundary line DL1 parallel to the XY plane and the boundary line DL2 orthogonal to the boundary line DL1Three regions 6a-6cDivided intoThe lattice spacing is different in each region. For this reason, the information reproducing light beam diffracted in the region 6a is applied to the light detector 7a of the photodetector 7 shown in FIG. 5, and the information reproducing light beam diffracted in the region 6b is applied to the light detector 7b. The information reproducing light beam diffracted in (1) is incident on the boundary lines of the light detection units 7c and 7d, respectively.The boundary line between the light detection units 7c and 7d is parallel to the XY plane orthogonal to the first to fourth surfaces 18, 16, 17, and 19.The information reproducing light beam transmitted through the first diffractive element 6 as the 0th-order diffracted light is incident on the light detection units 7e and 7f.
[0059]
The two tracking beams that have passed through the first diffractive element 6 as the 0th-order diffracted light are detected by the light detection units 7g and 7h, respectively.
[0060]
Accordingly, a focus error signal based on the Foucault method is obtained by calculating the difference between the output signals of the light detection units 7c and 7d, and the three beam method is calculated by calculating the difference between the output signals of the light detection units 7g and 7h. A radial error signal based on is obtained. Further, a so-called push-pull signal can be obtained by calculating the difference between the output signals of the light detection units 7a and 7b. This push-pull signal is used for detecting an address signal recorded by meandering a tracking groove formed on the magneto-optical recording medium 12, for example. The magneto-optical signal is obtained by calculating the difference between the output signals of the light detection units 7e and 7f.
[0061]
The refractive index of the second member 14 of the beam splitter 2 is preferably closer to the refractive index of the first member 15 because the refraction at the boundary surface is small and the generation of astigmatism can be suppressed. When the astigmatism increases, the shape of the focused spot on the photodetector 7 becomes disordered and becomes large, and there is a problem that the photodetector 7 must be enlarged accordingly. As the first member 15, glass is generally used, and its refractive index is 1.4 to 2.0. For this reason, it is desirable that the refractive index of the second member 14 is also 1.4 to 2.0. For example, quartz (Ne = 1.547, No = 1.539), sapphire (Ne = 1.760, No = 1.768) or lithium tetraborate (Ne = 1.605, No = 1.549) is preferably used. Note that Ne represents the refractive index for extraordinary light in birefringence, and No represents the refractive index for ordinary light in birefringence. In particular, lithium tetraborate has a large birefringence and can spatially separate two polarized light at a short distance. For this reason, the package 13 can be made compact, which is preferable.
[0062]
Although the half-wave plate 3 is not necessary, the reflectance of the polarizing film formed on the first surface 18 of the first member 15 of the beam splitter 2 is generally produced by increasing the reflectance of S-polarized light. It's easy to do. For this reason, when the polarization direction of the light emitted from the semiconductor laser 1 is P-polarized light, the half-wave plate 3 is preferably disposed in order to rotate the polarization direction by 90 °.
[0063]
In the optical pickup device described above, branching of reflected light and polarization separation can be realized by the beam splitter 2 configured by only two members (the first member 15 and the second member 14). For this reason, an apparatus can be reduced in size. Further, since the number of component parts of the beam splitter 2 is small, the manufacturing error is small, and the positional deviation of the light beam incident on the photodetector 7 is small. For this reason, the assembly of an apparatus is easy. Further, the birefringence of the crystal is used for polarization separation. For this reason, there is no difference in intensity between the two separated linearly polarized lights. Furthermore, since the extinction ratio is determined by crystallinity, an extinction ratio of about 1: 100 can be easily obtained by using a good crystal. For this reason, light utilization efficiency can be improved. Moreover, since the photodetector 7 does not require an LD mounting portion, the substrate becomes small, and the manufacturing cost is suppressed.
[0064]
The cross section of the first member 15 is a parallelogram. For this reason, it is possible to manage the parallelism and interval of the light incident on the first member 15 and the light emitted from the first member 15 with high accuracy. In addition, since the first member 15 is a parallelogram, a manufacturing method in which a large substrate is overlaid and then cut can be adopted as a method of manufacturing the beam splitter 19. For this reason, the mass productivity of the optical pickup device can be improved.
[0065]
Further, the first diffraction element 6 does not exist on the optical path from the semiconductor laser 1 to the magneto-optical recording medium 12. For this reason, since the light emitted from the semiconductor laser 1 is efficiently transmitted to the magneto-optical recording medium 12, the semiconductor laser 1 having a lower output can be used. Thereby, the downsizing of the apparatus and the degree of design freedom can be improved, and the mass productivity can be improved. Further, it is not necessary to consider that the diffracted light from the first diffractive element 6 does not enter the collimating lens 10 and the objective lens 11. For this reason, the freedom degree of design of the 1st diffraction element 6 improves. Further, since the diffraction angle of the first diffraction element 6 may be small, the grating pitch of the hologram can be kept large even when the wavelength of incident light is shortened. So, for example, denseArrivalThe first diffraction element 6 can be manufactured by a method such as an exposure method, mass productivity and processing accuracy can be improved, and an optical pickup device can be manufactured at low cost. Further, the light diffracted by the first diffractive element 6 is reflected by the magneto-optical recording medium 12 and is incident on the photodetector 7, thereby generating false signals in the focus error signal and the tracking error signal. Does not occur.
[0066]
Furthermore, the second diffractive element 5 is also formed on the light transmissive substrate 4 on which the first diffractive element 6 is formed. For this reason, a tracking signal can be obtained by a stable three-beam method without increasing the number of parts. As a result, the apparatus can be miniaturized and the light utilization efficiency can be increased. Further, since the second diffraction element 5 is formed in the same plane as the first diffraction element 6, the second diffraction element 5 and the first diffraction element 6 can be manufactured at the same time. For this reason, the number of manufacturing steps does not increase, and the mass productivity of the apparatus can be improved.
[0067]
Further, the half-wave plate 3 is arranged between the beam splitter 2 and the package 13. Therefore, a pickup can be configured regardless of the far field pattern and the polarization direction of the semiconductor laser 1, and a polarizing film formed on the first surface 18 of the beam splitter 2 can be easily manufactured.
[0068]
A material having a refractive index close to that of glass is used for the second member 14. For this reason, the refraction angle of the chief ray at the boundary between the first member 15 and the second member 14 becomes small, and astigmatism and coma generated can be suppressed.
[0069]
When lithium tetraborate having a large birefringence is used as the second member 14, the spatial distance of the polarized light can be increased.
[0070]
[Embodiment 2]
Referring to FIG. 6, the optical pickup device according to the embodiment of the present invention includes a stem 8, a semiconductor laser 1 that is a light source provided on the stem 8, a cap 9 that covers the stem 8, A light transmitting substrate 4 attached to the light transmitting substrate 4, a beam splitter 2 attached to the light transmitting substrate 4, a collimating lens 10 for condensing the light emitted from the semiconductor laser 1 on the magneto-optical recording medium 12, and The objective lens 11 and a photodetector 27 that is disposed on the stem 8 and detects reflected light from the magneto-optical recording medium 12 branched by the beam splitter 2 are included.
[0071]
The beam splitter 2 is disposed on the optical path from the semiconductor laser 1 to the collimating lens 10 and separates part of the reflected light from the magneto-optical recording medium 12 as described above. The beam splitter 2 is composed of a first member 15 made of an isotropic optical material and a second member 14 made of an anisotropic optical material, and the light from the semiconductor laser 1 is the first member 15. It passes through only the inside and reaches the collimating lens 10, and the reflected light from the magneto-optical recording medium 12 passes through the first member 15 and the second member 14 and reaches the photodetector 27. . The first member 15 constituting the beam splitter 2 is mainly a prism having a parallelogram in cross section, a first surface 18 in contact with the second member 14, and a second surface 16 adjacent to the first surface 18. And a third surface 17 facing the first surface 18 and a fourth surface 19 facing the surface of the second surface 16.
[0072]
The light transmissive substrate 4 is disposed on an optical path from the beam splitter 2 to the photodetector 27, and diffracts part of the reflected light from the magneto-optical recording medium 12 to generate a control signal. 26 is formed. Further, on the surface where the first diffractive element 26 is formed, the light emitted from the semiconductor laser 1 passes through the portion where the light from the semiconductor laser 1 passes two tracking beams and one information reproducing beam. The second diffraction element 25 is formed to be divided into a total of three beams.
[0073]
A window glass 21 is attached to the light passage region of the cap 9, and the inside is hermetically sealed. By sealing the inside of the package 23 composed of the stem 8 and the cap 9, the relative position between the semiconductor laser 1 and the photodetector 27 is kept stable.
[0074]
S-polarized light emitted from the semiconductor laser 1 is split by the second diffraction element 25 into a total of three beams, two tracking beams and one information reproducing beam. The three beams are incident on the first member 15 of the beam splitter 2 from the second surface 16. The three beams incident on the first member 15 are reflected by the third surface 17 and the first surface 18 and then emitted from the fourth surface 19, and are magneto-optical recording medium by the collimating lens 10 and the objective lens 11. 12 is condensed. The second diffractive element 25 is a linear grating formed on the same plane as the first diffractive element 26 of the light-transmitting substrate 4 as described above and having a constant interval as shown in FIG. The polarization characteristics of the first surface 18 include, for example, a reflectance of S-polarized light of 70% (transmittance of S-polarized light is 30%), and a reflectance of P-polarized light is 0% (transmittance of P-polarized light is 100%). The magneto-optical recording medium 12 is irradiated with 70% of the light emitted from the semiconductor laser 1.
[0075]
The plane of polarization of the light reflected by the magneto-optical recording medium 12 rotates according to the direction of magnetization recorded on the magneto-optical recording medium 12. The light whose polarization plane has rotated passes through the objective lens 11 and the collimating lens 10 and enters the fourth surface 19 of the first member 15 of the beam splitter 2. The light incident on the fourth surface 19 passes through the first surface 18 and enters the second member 14. As described above, the polarization characteristics of the first surface 18 are, for example, that the reflectance of S-polarized light is 70% (the transmittance of S-polarized light is 30%), and the reflectance of P-polarized light is 0% (the transmittance of P-polarized light is 100). %). For this reason, when the light reflected by the magneto-optical recording medium 12 passes through the first surface 18, the amount of rotation of the polarization plane apparently increases.
[0076]
Since the second member 14 has optical anisotropy, the reflected light from the magneto-optical recording medium 12 in the second member 14 is separated into two orthogonal polarization components and travels in different directions. The reflected light that has been polarized and separated is incident on the first diffractive element 26, and a part thereof is diffracted. Since three light beams are reflected from the magneto-optical recording medium 12, a total of six light beams are incident on the first diffraction element 26 by polarization separation.
[0077]
With reference to FIG. 8, the shape of the first diffraction element 26 and the six light beams incident thereon will be described. Solid and dashed circles indicate light separated into two orthogonal polarization components. The first diffraction element 26 has three regions 26a to 26c, and the lattice spacing is different in each region. For this reason, the information reproducing light beam diffracted in the region 26a is transmitted to the light detection unit 27a of the photodetector 27 shown in FIG. The information reproducing light beam diffracted in (1) is incident on the boundary lines of the light detection units 27c and 27d, respectively. The information reproducing light beam that has passed through the first diffractive element 26 as the 0th-order diffracted light is incident on the light detection units 27f and 27i.
[0078]
One of the two tracking beams that have passed through the first diffraction element 26 as the 0th-order diffracted light is detected by the light detection units 27e and 27g, and the other is detected by the light detection units 27h and 27j.
[0079]
Therefore, by calculating the difference between the output signals of the light detection units 27c and 27d, a focus error signal based on the Foucault method is obtained, and the sum of the output signals of the light detection units 27e and 27g and By calculating the difference from the sum of the output signals, a radial error signal based on the three beam method can be obtained. Also, a so-called push-pull signal can be obtained by calculating the difference between the output signals of the light detection units 27a and 27b. This push-pull signal is used, for example, to detect an address signal recorded by meandering a tracking groove formed on the magneto-optical recording medium 12. The magneto-optical signal is obtained by calculating the difference between the output signals of the light detection units 27f and 27i.
[0080]
The refractive index of the second member 14 of the beam splitter 2 is preferably closer to the refractive index of the first member 15 because the refraction at the boundary surface is small and the generation of astigmatism can be suppressed. When the astigmatism increases, the shape of the focused spot on the photodetector 27 becomes disordered and increases, and the photodetector 27 has to be increased accordingly. As the first member 15, glass is generally used, and its refractive index is 1.4 to 2.0. For this reason, it is desirable that the refractive index of the second member 14 is also 1.4 to 2.0. For example, quartz (Ne = 1.547, No = 1.539), sapphire (Ne = 1.760, No = 1.768) or lithium tetraborate (Ne = 1.548, No = 1.604) is preferably used. In particular, lithium tetraborate has a large birefringence and can spatially separate two polarized light at a short distance. For this reason, the package 23 can be made compact, which is preferable.
[0081]
A material is selected such that the refractive index of the first member 15 made of an isotropic material constituting the beam splitter 2 is equivalent to the extraordinary light refractive index Ne of the second member 14 made of an anisotropic optical material. By doing so, it is possible to increase the polarization separation angle when the first surface 18 performs polarization separation, which is preferable. In other words, the difference between the refractive index of the first member 15 and the extraordinary refractive index Ne of the second member is ½ of the difference between the ordinary refractive index No and the extraordinary refractive index Ne of the second member. It is desirable to select a material having a refractive index that falls within the range as the first member 15. In general, the refractive index of a birefringent material can be considered based on the refractive index ellipsoid. Is done.
[0082]
For example, as the first member 15 and the second member 14, optical glass SF2 manufactured by SCHOTT (N = 1.63553, N is a refractive index) and lithium tetraborate (Ne = 1.548, No = 1. 604), the angle formed by the polarization direction of the incident light on the second member 14 and the crystal axis of the second member 14 is set to 45 °. The extraordinary refractive index Ne ′ of the incident light in the second member 14 is sequentially calculated from the relationship between the polarization direction of the incident light on the second member 14 and the crystal axis of the second member 14. Is 1.563.
[0083]
Further, as the first member 15 and the second member 14, SF2 (N = 1.63553) and lithium tetraborate (Ne = 1.548, No = 1.604) are used, respectively, and the second member 14 is used. The angle formed between the polarization direction of the incident light and the crystal axis of the second member 14 is set to 0 °. The extraordinary refractive index Ne ′ of the incident light in the second member 14 is 1.548. Therefore, the polarization separation of extraordinary light at the boundary surface between the first member 15 and the second member 14 depends on the angle between the polarization direction of the incident light on the second member 14 and the crystal axis of the second member 14. The angle changes. This phenomenon is called walk-off.
[0084]
Here, three examples of the phenomenon in which the polarization separation angle changes depending on the material constituting the beam splitter 2 will be described.
[0085]
As a first example, SF2 (N = 1.63553) is used as the glass material that is the first member 15 constituting the beam splitter 2, and lithium tetraborate (Ne) is used as the birefringent material that is the second member 14. = 1.548, No = 1.604). Further, when the angle formed by the polarization direction of the incident light to the second member 14 and the crystal axis of the second member 14 is set to 45 °, the above walk-off occurs. For this reason, the polarization separation angle at the boundary surface between the first member 15 and the second member 14 is about 1.597 °. In this case, the difference between the refractive index of SF2 and the extraordinary refractive index of lithium tetraborate is large. Also, extraordinary light has greater aberration than ordinary light. This makes optical design difficult.
[0086]
As a second example, a glass material having a refractive index equivalent to the extraordinary refractive index Ne of the birefringent material that is the second member 14 is used for the first member 15. For example, optical glass PSK3 (N = 1.547) manufactured by SCHOTT is used for the first member 15, and lithium tetraborate (Ne = 1.548, No = 1.604) is used for the second member 14. Use. When the angle formed by the polarization direction of the incident light on the second member 14 and the crystal axis of the second member 14 is set to 45 °, the walk-off is suppressed, and the polarization separation angle at the boundary surface is about 2.times. 001 °. Therefore, by using a glass material having a refractive index equivalent to the extraordinary refractive index of the second member 14 as the first member 15 rather than using SF2 (N = 1.63553) for the first member 15. The polarization separation angle can be increased.
[0087]
Referring to FIG. 10, the crystal axis direction of the birefringent material used for second member 14 is an intersection line of third surface 17 and fourth surface 19 in the same plane as fourth surface 19. For 2a, it is set to 45 °. Thereby, the magneto-optical signal contained in the light reflected by the magneto-optical recording medium 12 is stably separated.
[0088]
As a third example, the difference between the extraordinary light refractive index of the birefringent material that is the second member 14 and the refractive index of the glass material that is the first member 15 is the ordinary refractive index and extraordinary light of the second member 14. A glass material that is ½ or less of the difference from the refractive index is used. For example, optical glass LF5 (N = 1.722) manufactured by SCHOTT is used for the first member 15, and lithium tetraborate (Ne = 1.548, No = 1.604) is used for the second member 14. Use. Further, when the angle formed by the polarization direction of the incident light on the second member 14 and the crystal axis of the second member 14 is set to 45 °, the first member 15 and the second member 15 are slightly affected by the above-mentioned walk-off. The polarization separation angle at the interface with the member 14 is about 1.98 °. In this case, ordinary light and extraordinary light have similar aberrations. For this reason, optical design becomes easy.
[0089]
In the optical pickup device described above, the first member 15 and the second member 15 have the same refractive index as that of the extraordinary light refractive index of the second member 14 made of an anisotropic optical material. By selecting the member 14, the polarization separation angle on the first surface 18 can be increased. Therefore, the height of the package 23 can be reduced, and the apparatus can be reduced in size. Further, since the influence of the walk-off can be suppressed, the optical design can be easily performed.
[0090]
The direction of the crystal axis of the second member 14, which is an anisotropic optical material, is 45 ° in the same plane as the intersection line 2 a of the third surface 17 and the fourth surface 19 and the fourth surface 19. The beam splitter 2 is configured. For this reason, the magneto-optical signal contained in the light condensed on the magneto-optical recording medium 12 by the collimating lens 10 and the objective lens 11 can be stably separated.
[0091]
The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.
[Brief description of the drawings]
FIG. 1 is a side external view showing a configuration of an optical pickup device according to a first embodiment of the present invention.
FIG. 2 is an external view of a second diffraction element 5;
FIG. 3 is a diagram for explaining a change in the amount of rotation of a polarization plane of light before and after passing through a first surface;
FIG. 4 is a diagram for explaining the shape of the first diffractive element 6 and six light beams incident on the first diffractive element 6;
FIG. 5 is a diagram for explaining the shape of the photodetector 7 and the light beam incident on the photodetector 7;
FIG. 6 is a side external view showing a configuration of an optical pickup device according to a second embodiment of the present invention.
7 is an external view of a second diffraction element 25. FIG.
FIG. 8 is a diagram for explaining the shape of the first diffraction element and six light beams incident on the first diffraction element.
9 is a diagram for explaining the shape of the photodetector 27 and the light beam incident on the photodetector 27. FIG.
10 is a top view of the beam splitter 2 for explaining the crystal axis of the second member 14. FIG.
FIG. 11 is a side external view showing a configuration of a conventional first magneto-optical pickup device.
FIG. 12 is a top view of a light receiving element, a light emitting element, and an analyzer portion of a first conventional magneto-optical pickup apparatus.
FIG. 13 is a side external view showing a configuration of a second conventional magneto-optical pickup apparatus.
FIG. 14 is a top view of a light receiving element and a light emitting element part of a second conventional magneto-optical pickup apparatus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Semiconductor laser, 2 Beam splitter, 3 1/2 wavelength plate, 4 Light transmissive board | substrate, 5,25 2nd diffraction element, 6,26 1st diffraction element, 7, 27 Photodetector, 8 stem, 9 Cap, 10 collimating lens, 11 objective lens, 12 magneto-optical recording medium, 13, 23 package, 14 second member, 15 first member.

Claims (11)

A light source;
A lens disposed on an optical path from the light source to the magneto-optical recording medium;
A beam splitter disposed on an optical path from the light source to the lens and separating a part of reflected light from the magneto-optical recording medium;
A photodetector for detecting the reflected light separated by the beam splitter ;
A first diffractive element disposed on an optical path from the beam splitter to the photodetector ,
The beam splitter is
A first member made of an isotropic optical material, reflecting light from the light source to reach the magneto-optical recording medium , and allowing reflected light from the magneto-optical recording medium to pass through;
Wherein adjacent the first member, made of optically anisotropic material, seen including a second member for further passing the reflected light from the magneto-optical recording medium which has passed through the first member,
The first member has a parallel four-sided cross section having a first parallel surface facing each other and a second parallel surface facing each other and intersecting each of the first parallel surfaces at a predetermined angle. The other of the second parallel surfaces so that one of the first parallel surfaces is in contact with the second member, and one of the second parallel surfaces is opposed to the light source. Are respectively arranged to face the lens,
The predetermined angle is emitted from the light source, and light incident on the one of the second parallel surfaces at a predetermined incident angle is generated between the other of the first parallel surfaces and the first parallel surface. Are selected to be reflected in this order by the one and exit from the other of the second parallel surfaces;
The photodetector includes a set of two-divided photodetectors that are divided into two by a boundary line parallel to a plane orthogonal to the first and second parallel planes of the beam splitter,
The first diffractive element has first and second regions divided into two by a parting line parallel to a plane orthogonal to the first and second parallel surfaces of the beam splitter;
The optical pickup device , wherein the reflected light from the magneto-optical recording medium diffracted in the first region is guided onto a boundary line of the two-divided light detection unit . The optical pickup device according to claim 1, wherein the first member has a refractive index that is substantially the same as an extraordinary refractive index of the second member. In the first member, the difference between the refractive index of the first member and the extraordinary light refractive index of the second member is 1 of the difference between the ordinary light refractive index and the extraordinary light refractive index of the second member. The optical pickup device according to claim 1, wherein the optical pickup device has a refractive index that is less than or equal to / 2. The crystal axis of the second member is orthogonal to the light emitted from the other of the second parallel surfaces, and the direction vector of the direction of the light emitted from the other of the second parallel surfaces, and the The optical pickup device according to claim 3, wherein the optical pickup device is selected so as to form approximately 45 ° with respect to a plane including the one normal vector of the first parallel plane. A light transmissive substrate provided between the light source and the photodetector and the beam splitter ;
The optical pickup device according to claim 1, wherein the first diffraction element is provided on the light transmissive substrate . 6. The light according to claim 5 , further comprising a second diffractive element provided at a position in the light transmissive substrate that receives light from the light source and that splits the light from the light source into three or more light beams. Pickup device. The optical pickup device according to claim 6 , wherein the first and second diffraction elements are juxtaposed on the same plane. The optical pickup device according to claim 1, further comprising a half-wave plate provided between the light source and the beam splitter. The optical pickup device according to claim 1, wherein the second member has a refractive index of 1.4 to 2.0. The optical pickup device according to claim 9 , wherein the second member is made of lithium tetraborate. The optical pickup device according to claim 1, wherein a focus error signal is obtained by comparing outputs of the two-divided detection unit.

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